. Figure 1 – Hydrocyclones The separation is not perfect, the time in the cyclone is short and fines can be entrained with the underflow, a serious drawback is that it is a poor classifier where the cyclones are fed with high pulp densities. To control the cyclone, it should be fed with a constant flow and the resistance to flow from the Apex and vortex require a pressure loss of 40 to 150kPa. The separation is affected by the diameter of the cyclone, the ratio of coarse to fine material, the density of the feed and the density of the solids in the slurry. Smaller cyclones cut at a finer point than larger diameters and usually multiple cyclones are required to treat the full flow. It may be necessary to use two stages of cyclones with interstage pumping and dilution to achieve the desired separation. Cyclone sand is generated continuously over the life of the tailings dam creating progressive dam raises. The cyclones can be installed at equidistant points on the crest of the dam or in a central cyclone station where the underflow is pumped to spigots along the dam crest. The construction of tailings dams using hydrocyclone sands is a successful and efficient methodology. • But with increasing water scarcity, can we improve? By treating tailings from the underflow of a hydrocyclone in a dewatering screen, clays and ultrafine particles are effectively removed, improving drainage and water recovery. Cyclones of a nominal diameter around 500 mm are commonly used where depending upon the tailing’s characteristics 40% to 50% of the feed reports to the underflow producing a thick stream with up to 75% solids. This material flows down the dam’s outer wall in a stream. The underflow tails do not spread well, and the discharge point is moved between multiple points along the wall to control the deposition. Once deposited and drained the material is compacted and spread with earth moving equipment to achieve the required compaction specifications. These specifications depend on the dam design but typically the sand is compacted in thin layers, to a density higher than 95% of Standard Maximum Dry Density. The material continues to increase in density as it drains and is exposed to consolidation, dry density of 1.67 g/cm³ have been reported for tails with an sg of 2.6, changes in sg and size can vary the compacted density. Hydraulic transport The transport of tailings in a slurry is extremely cost effective when sufficient altitude is
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